High-efficiency video coding (HEVC) is the latest video coding standard developed by the Joint Collaborative Team on Video Coding (JCT-VC). In HEVC, a coded picture is represented by a plurality of coded slices. Each slice is partitioned into multiple coding tree units (CTUs). In the main profile, the minimum and the maximum sizes of CTU are specified by the syntax elements in the sequence parameter set (SPS), where the CTU size can be 8×8, 16×16, 32×32, or 64×64. Inside a slice, a raster scan method is used for processing the CTU accordingly. The CTU is further partitioned into multiple coding units (CUs) to adapt to various local characteristics. Each CU can be further divided into one or more non-overlapped prediction units (PUs). The PU works as a basic operation unit for predicting sample values in a block region. After obtaining the residual block by the prediction process, a CU can be partitioned into transform units (TUs). The TU is a basic block for applying transform and quantization operation for representing the prediction residual signal from a block region. A TU can consist of multiple transform blocks (TBs) corresponding to different color components from the same block region.
Joint Video Exploration Team (JVET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11 are currently developing the next-generation international video coding standard. In Joint Exploration Test Model 4 (JEM4) as specified in JVET-D1001 (J. Chen, et al, “Algorithm Description of Joint Exploration Test Model 4,” Joint Video Exploration Team of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG11, JVET-D1001, 4th Meeting: Chengdu, C N, 15-21 Oct. 2016), Adaptive Multiple core Transform (AMT) is adopted for providing additional transform types for coding residual blocks. For HEVC, the DCT-2 (Discrete Cosine Transform-2) transform type is used. In JVET-D1001, additional transform types including DCT-8 and DST-7 (Discrete Sine Transform-7) are also supported. The additional transform set can only be applied to luma transform blocks. When the luma TB is signaled to be non-zero, an on/off control flag (i.e., emt_cu_flag) is further signaled to indicate whether AMT is applied to the luma TBs in the current CU. When emt_cu_flag is equal to 1, the selected transform index information is further coded. According to JVET-D1001, AMT is not applied to the chroma TBs and DCT-2 is always used by the chroma TBs.
In JVET-E0036 (T. Tsukuba, et al, “On Adaptive Multiple Core Transform for Chroma,” Joint Video Exploration Team of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG11, JVET-E0036, 5th Meeting: Geneva, CH, 12-20 Jan. 2017), AMT is also applied to chroma TBs. However, there is no separate signaling for the chroma TBs. Therefore, the signaled AMT operation for the luma TBs is also applied to the chroma TBs from the same TU. The AMT for chroma TBs based on JVET-E0036 has shown some coding performance improvement.
It is desirable to further improve the coding performance of AMT on the chroma TBs. Accordingly, various AMT signaling methods for the chroma TBs are disclosed.